Temperature compensated oscillators are well known in the art as described in U.S. Pat. Nos. 3,713,033; 4,297,657; 4,305,041; and 4,513,259; which are incorporated herein by reference. However, poor short-term stability has been a problem for the entire class of externally compensated crystal oscillators, of which the time compensated clock oscillator (TCCO) is a member.
In a basic TCCO, the frequency of a voltage controlled crystal oscillator (VCXO) is sampled at one to ten second intervals by a frequency counter. The frequency counter uses a reference frequency from a reference oscillator as its time base. A temperature correction programmable read only memory (PROM) stores reference frequency errors as a function of temperature. A microprocessor may receive inputs from a temperature sensor, the temperature correction PROM, the reference oscillator, and the frequency counter to correct the output frequency for frequency errors and drift rate of the VCXO. The output frequency of the VCXO is corrected by a digital-to-analog (D/A) converter that provides a control voltage to the control input of the VCXO. However, short-term instability remains because the corrections are made only at specific sample times, which causes the VCXO to provide a sawtooth-shaped frequency change with respect to time. This occurs because the VCXO drifts without a control voltage change until the next sample time, at which time a new control voltage is provided.
Because prior TCCOs have short-term instability, there is a need for an improved TCCO for applications requiring precise carrier frequency control. An improved TCCO should provide a control voltage to the VCXO to provide continuous compensation for the frequency drift of the VCXO.